Redirect everyone but Feedburners spider to your feed.

  $HTTP["useragent"] !~ "FeedBurner" {
    url.redirect = (
      "/atom.xml" => "http://feeds.feedburner.com/example",
      "/rss.xml" => "http://feeds.feedburner.com/example",
      "/index.xml" => "http://feeds.feedburner.com/example",
     )
  }